Dogwood trees are an interesting bunch. They grow on several different continents in many different climates, from the snowy Chinese mountains to the humid Eastern US.
One would think Cornus florida, native to the east coast, would like hot muggy conditions. Wrong. Too much moisture - humidity or fog - can lead to mildew. The trees will live, bloom, and grow except the leaves shrink and curl up with black. Definitely unsightly.
One course of action is to spray them with a specialty fungicide. Dow AgroSciences makes Eagle 20EW [active ingredient myclobutanil] that wards off mildew.
There are hybrids, like Cherokee Brave, that have been bred to be more resistant to mildew. The main thing to remember is Dogwoods are worth the fuss!